1. Field of the Invention
This invention relates to a process for preparing diacetylbenzene from diethylbenzene, ethylacetophenone or a mixture thereof by a liquid phase oxidation reaction using a molecular oxygen-containing gas.
2. Description of the Prior Art
Diacetylbenzene is used as an intermediate, for example, for preparing divinylbenzene which is a cross-linking agent for high molecular weight polymers, by a hydrogenation and a dehydration, and for preparing an aromatic diamine by an oximation and a Beckman rearrangement. Also, diacetylbenzene is a starting material for preparing heat resistant polymers and one of the promising compounds for the chemical industry.
In general, in the liquid phase oxidation of an ethyl group bonded to a benzene nucleus, the corresponding carboxylic acid is easily produced. Accordingly, in order to selectively produce an acetyl substituted benzene, which is an intermediate compound in oxidizing the ethyl substituted benzene to the corresponding carboxylic acid, not only are there restrictions on the starting materials, but also it is necessary to take special measures for selecting catalysts and reaction conditions.
Furthermore, in the production of diacetylbenzene which is a difunctional compound the following further restrictions on the reaction conditions and purification methods in addition to those mentioned above are imposed.
A first restriction originates from the reactivity of ethylacetophenone. In general, in the autoxidation reaction of an alkylbenzene the reactivity is in accordance with the Hammett rule and its reaction constant .rho. is a negative value. Thus, it is more difficult to oxidize the ethyl group of ethylacetophenone whose acetyl group exhibits a strong electron attraction, and the reaction conditions become more rigorous. As a result, the selectivity to diacetylbenzene tends to decrease, and more severe restrictions on the selection of catalysts and reaction conditions are imposed.
A second restriction arises from the need to suppress the formation of compounds which hinder the oxidation reaction. Namely, in the liquid phase oxidation reaction, compounds which hinder the oxidation reaction are formed as the oxidation reaction proceeds, and the oxidation reaction tends to stop. For this reason the oxidation reaction is carried out at a low conversion in most cases. Thus, when diacetylbenzene is prepared from diethylbenzene at a low conversion, the yield per volume of the reactor is reduced and the quantity of heat for separating the products from the starting material is remarkably increased. Furthermore, since the oxidation reaction of diethylbenzene to diacetylbenzene is a sequential reaction through ethylacetophenone as an intermediate, at low conversions, the oxidation reaction stops at the ethylacetophenone stage, and as a result very little diacetylbenzene is formed. Thus, in this oxidation reaction the formation of compounds which hinder the oxidation reaction must be suppressed to achieve a high conversion.
A third restriction arises in the purification of diacetylbenzene. Since diacetylbenzene is a difunctional compound having a high boiling point, the decomposition and polymerization of diacetylbenzene take place when diacetylbenzene is distilled in the presence of the catalyst, and therefore the catalyst must be removed prior to the distillation of the diacetylbenzene.
A fourth restriction is due to the selectivity to diacetylbenzene. Since diacetylbenzene is difunctional compounds, various by-products can be formed, and some of them have a boiling point very close to each other. Thus, since it becomes more difficult to obtain a product of high purity, the selectivity to the products must be high.
Accordingly, in preparing diacetylbenzene from diethylbenzene as a starting material on an industrial scale, the purification system must be taken into account in addition to the oxidation reaction system. More specifically, a technique must be established in which the oxidation reaction at a high conversion of the starting material can be achieved while the formation of compounds hindering the oxidation reaction is suppressed, the selectivity to the product at a high conversion of the starting material is increased, and a regenerable catalyst can be efficiently separated from the reaction mixture liquid.
It is known that diacetylbenzene can be prepared by the liquid phase oxidation of diethylbenzene as a starting material using an oil-soluble metal salt catalyst such as a naphthenate of a transition metal. However, according to this method the decomposition of diacetylbenzene partially takes place in the separation procedure using a distillation after the oxidation reaction and it is difficult to obtain the product in high purity with high yield. On the other hand, Japanese Patent Application (OPI) 72231/1974 describes a method of purifying diacetylbenzene which comprises treating the reaction mixture, which has been obtained by the liquid phase oxidation of diethylbenzene using oxygen or an oxygen-containing gas and which contains, as the main component, diacetylbenzene, as such or after a flash distillation, with an alkali and distilling the reaction mixture thus treated. When the catalyst is removed according to this invention, it is necessary to provide another step of regenerating the recovered catalyst for reuse.